Low pressure steam turbine including pivotable nozzle

ABSTRACT

A low pressure steam turbine is disclosed including a rotor having: a rotor body; and a plurality of bucket stages disposed axially along the rotor body, the plurality of bucket stages including a last bucket stage; and a nozzle assembly at least partially surrounding the rotor, the nozzle assembly including a plurality of nozzle stages corresponding to the plurality of bucket stages, wherein a nozzle in a nozzle stage axially upstream of the last bucket stage is adjustable to modify a fluid flow within the last bucket stage during operation of the low pressure steam turbine.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to a low pressure steam turbine system including pivotable (or, variable area) steam turbine nozzles (or, guidevanes). Specifically, the subject matter disclosed herein relates to a low pressure steam turbine having variable nozzles with variable operating areas in its low pressure section for improving the efficiency or extending the operating envelope of the steam turbine system.

Steam turbine power systems are designed and built with particular load conditions in mind. Often, these systems are designed and optimized to handle the peak or near-peak loads of their customers, and/or coincide with average day ambient temperatures and condenser backpressures. These conditions can typically drive selection of large last stage vane (or, bucket) annulus area in the steam turbine low pressure section. However, during periods of lower demand, higher ambient temperatures, or higher condenser backpressures, these systems must run at off-peak conditions. For example, a steam turbine power system may reduce its output to well below fifty percent of its rated power during the evening hours (e.g., after 9:00 pm local time), when customers require very little electricity. Reducing the output of the steam turbine power system to such levels may cause, among other things, system inefficiencies (e.g. low pressure section exhaust losses) and mechanical integrity concerns (e.g. for last stage buckets), as the steam turbine is not designed for these conditions.

BRIEF DESCRIPTION OF THE INVENTION

Aspects of the invention provide for a low pressure steam turbine including a nozzle assembly having variable area nozzles in the low pressure section. In one embodiment, the steam turbine includes: a rotor having: a rotor body; and a plurality of bucket stages disposed axially along the rotor body, the plurality of bucket stages including a last bucket stage; and a nozzle assembly at least partially surrounding the rotor, the nozzle assembly including a plurality of nozzle stages corresponding to the plurality of bucket stages, wherein a nozzle in a nozzle stage axially upstream of the last bucket stage is adjustable to modify a fluid flow within the last bucket stage during operation of the steam turbine.

A first aspect of the invention includes a low pressure steam turbine having: a rotor having: a rotor body; and a plurality of bucket stages disposed axially along the rotor body, the plurality of bucket stages including a last bucket stage; and a nozzle assembly at least partially surrounding the rotor, the nozzle assembly including a plurality of nozzle stages corresponding to the plurality of bucket stages, wherein a nozzle in a nozzle stage axially upstream of the last bucket stage is adjustable to modify a fluid flow within the last bucket stage during operation of the steam turbine.

A second aspect of the invention includes a low pressure steam turbine system having: a rotor having: a diffuser section; a turbine coupled to the diffuser section, the turbine including: a rotor body; and a plurality of bucket stages disposed axially along the rotor body, the plurality of bucket stages including a last bucket stage; a nozzle assembly at least partially surrounding the rotor, the nozzle assembly including a plurality of nozzle stages corresponding to the plurality of bucket stages, wherein a nozzle in a nozzle stage axially upstream of the last bucket stage is adjustable to modify a fluid flow within the last bucket stage or the diffuser during operation of the steam turbine; and a control system operably connected to the nozzle stage, the control system configured to actuate pivoting of the nozzle in response to a predetermined load condition.

A third aspect of the invention includes a low pressure steam turbine apparatus having: a diffuser section; a turbine coupled to the diffuser section, the turbine including: a rotor having: a rotor body; and a plurality of bucket stages disposed axially along the rotor body, the plurality of bucket stages including a last bucket stage; and a stator substantially surrounding the rotor, the stator including: a nozzle assembly at least partially surrounding the rotor, the nozzle assembly including a plurality of nozzle stages corresponding to the plurality of bucket stages, wherein a nozzle in a nozzle stage axially upstream of the last bucket stage is pivotable about an axis to modify a fluid flow within the last bucket stage or the diffuser during operation of the steam turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which:

FIG. 1 shows a two dimensional cross-sectional view of a portion of a steam turbine according to embodiments of the invention.

FIG. 2 shows a three-dimensional perspective view of a portion of a steam turbine nozzle stage according to embodiments of the invention.

FIG. 3 shows a schematic view of a steam turbine system according to embodiments of the invention.

FIG. 4 shows a two-dimensional cross-sectional view of a portion of a low pressure steam turbine last stage according to embodiments of the invention.

FIG. 5 shows an illustrative exhaust loss graph for reference purposes.

It is noted that the drawings of the invention are not necessarily to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide for a steam turbine system including variable area (e.g., pivotable) steam turbine nozzles (or, guidevanes). Specifically, the subject matter disclosed herein relates to a steam turbine having variable area nozzles in its low pressure section for improving the efficiency or mechanical life at off-design loads of the steam turbine system.

Steam turbine power systems are designed and built with particular load conditions in mind. Often, these systems are built to handle the peak or near-peak loads of their customers, which may coincide with afternoon hours where the ambient temperature is high (e.g., above 27 degrees Celsius). However, during periods of lower demand, these systems must run at off-peak loads. For example, a steam turbine power system may reduce its output to well below fifty percent of its rated power during the evening hours (e.g., after 9:00 pm local time), when customers require very little electricity. Reducing the output of the steam turbine power system to such levels may cause, among other things, system inefficiencies, as the steam turbine is not designed with these conditions in mind. Additionally, inefficiencies may occur in the steam turbine when high back-pressure is generated in the condenser, such as on days when the ambient temperature is particularly high and the condenser runs above its designed conditions.

More specifically, during any of the aforementioned scenarios, the axial space upstream or downstream of the steam turbine's last bucket stage may experience fluid-dynamic conditions that cause inefficient rotation of the last-stage buckets in that stage, or turn-up losses in the exhaust (or, diffuser). That is, reduced steam flow or high back pressure causes less steam flow (and/or irregular flow) through this axial space, which is traditionally filled by steam flow under design conditions. This reduced/irregular steam flow generates vortex effects, irregular flow patterns and substantially stagnant regions, which may disrupt the intended movement of steam through the last stage buckets, and impair the diffusion by the exhaust downstream of the last stage bucket. This disruption may affect the torque generated by the rotor body, and subsequently, the output of the steam turbine (e.g., the low pressure steam turbine).

Prior attempts to address these issues have implemented flow modifiers (e.g., fins, louvers, baffles) axially downstream of the last bucket stage. That is, these prior attempts have tried to modify the flow of steam axially downstream of the last stage buckets in order to address the irregular flow initiated axially upstream of the last stage bucket.

In contrast to these prior attempts, aspects of the invention allow for modification of the steam flow path axially upstream of the last stage buckets using at least one adjustable (e.g., variable area) nozzle stage, which can be adjusted during operation of the steam turbine including the last stage nozzle. It is understood that as used herein, the term “variable area nozzle” may refer to a nozzle that has a fluid-facing surface with an adjustable angle and/or a adjustable surface area with respect to the fluid flow. That is, the nozzle's axially upstream-facing surface angle may be variable, e.g., via mechanical manipulation of the nozzle. It is understood that this mechanical manipulation may be achieved in a number of ways described herein. For example, each variable-area nozzle may be pivotable, rotatable, slideable, foldable, etc. about an axis or pivot point such that at least one fluid facing surface has a modifiable angle. In some cases, the nozzle airfoil itself may be pivotable, or nozzle-sidewall couplings may be pivotable about a particular axis or pivot point. Additionally, the nozzle airfoil may be segmented such that one or more segments pivot about one or more axes to modify the flow profile (speed, direction, etc.) across the airfoil or modify the fluid passing area between the nozzle and adjacent nozzle. These adjustments in the nozzle's effective surface area can be performed during operation of the steam turbine (in particular, in operation of the low-pressure steam turbine section).

Turning to FIG. 1, a cross-section of a portion of a low pressure steam turbine 10 is shown according to embodiments of the invention. As shown, the low pressure steam turbine 10 may include a rotor 12 having a rotor body 14, and a stator 16 having a nozzle assembly (where one nozzle stage 18 of the nozzle assembly is illustrated) at least partially surrounding the rotor body 14. The nozzle assembly (including stage 18) may have an inner diaphragm segment 20 and an outer diaphragm segment 22, which may take the form of diaphragm rings, as is known in the art. The diaphragm segments 20, 22 may hold (e.g., via mechanical connection such as welded bonding, jointing, or other suitable connection) a nozzle airfoil (e.g. partition) 24 in position to guide a working fluid (e.g., steam) across a plurality of turbine buckets 26 (one depicted in this view). As is known in the art, the turbine bucket 26 may be one of a plurality of buckets in a particular bucket stage of a turbine. A last bucket stage 28 is shown herein, where the last bucket stage 28 is located axially downstream (closer to the lower-pressure portion) of the steam turbine 10 than the nozzle 24.

As shown, the nozzle 24 (including the nozzle airfoil and/or sidewalls 25, 27) may be pivotable about an axis, thereby allowing the nozzle 24 to modify the flow of fluid within the last bucket stage 28 (e.g., across the bucket 26), across a space 29 axially upstream of the bucket 26. That is, the nozzle 24, including one or more sidewalls 25, 27, may be configured to pivot about an axis to modify the area between adjacent nozzle airfoils, thereby altering the fluid flow axially, radially, and/or circumferentially across the face of the nozzle 24. FIG. 1 depicts three illustrative axes about which the nozzle 24 may pivot. In one embodiment, the nozzle 24 may pivot about an axis (i), which run substantially perpendicular with the primary axis (a) of the rotor body 14. Axis (i) may intersect the primary axis (a) such that it spans radially from the primary axis (a) of the rotor body 14. In another embodiment, the nozzle 24 may pivot about an axis (ii), which runs substantially parallel with the primary axis (a) of the rotor body 14. In yet another embodiment, the nozzle 24 may pivot about an axis (iii, into and out of the page) tangential to a circumference of the stator 16. In this case, the nozzle 24 may pivot about an axis (iii) formed by a portion of a circular shape (or substantially circular shape) concentric with a circumference of the stator 16.

It is understood that although FIG. 1 depicts a single nozzle stage 18 and a single bucket stage 28, that the low pressure steam turbine 10 may include a plurality of nozzle stages and corresponding bucket stages. The plurality of nozzle stages may be configured to direct a working fluid (e.g., steam) across the buckets (e.g., bucket 26) in order to force rotation of those buckets, and consequently, rotation of the rotor body (or, shaft) 14 about its primary axis (a).

Turning to FIG. 2, a three-dimensional perspective view of a portion of a steam turbine nozzle stage 28 is shown according to another embodiment of the invention. It is understood that commonly numbered elements between the figures may indicate substantially identical components. Redundant explanation of these components has been omitted herein for clarity.

As shown in FIG. 2, the nozzle stage 28 may include a plurality of nozzles 24 (and/or nozzles 34) configured to be arranged circumferentially about a rotor (rotor not shown). As is known in the art, each nozzle stage 18 may be formed from upper and lower diaphragm segments, which may be joined at a horizontal joint surface of the steam turbine. Illustration of the interaction between upper and lower diaphragm segments is omitted herein for clarity, however, it is understood that the portion of the nozzle stage 18 shown in FIG. 2 may be a portion of a lower (or upper) diaphragm segment. In any case, as illustrated in FIG. 2, at least one nozzle 34 of a plurality of nozzles may include a multi-segmented body having a first segment 34A and a second segment 34B operably connected to the first segment 34A (e.g., along the axis (i)). In some cases, the first segment 34A is configured to pivot relative to the second segment 34B about the axis (e.g., axis (i)). In this embodiment, the nozzle 34 may be hinged (e.g., via conventional pins, joints, perforations, etc.) such that the first segment 34A may move relative to the second segment 34B. In some cases, the sidewalls 25, 27 may include oversized slots proximate the first segment 34A such that the first segment has a range of motion of approximately plus or minus 30 degrees within the sidewalls 25, 27. In this embodiment, the position of second segment 34B may be fixed, while the position of first segment 34A may be alterable via movement of the first segment 34A. However, in other cases, the first segment 34A may be fixed, while the position of the second segment 34B may be alterable via movement of the second segment 34B.

It is understood that in other embodiments, one or more nozzles (e.g., nozzles 24 and/or 34) may be configured to pivot as a single unit, such that segments (e.g., segments 34A, 34B) are eliminated. In this case, the nozzles (e.g., nozzles 24 and/or 34) may pivot within oversized slots in the sidewalls 25, 27, along any axis (e.g., (i), (ii) and/or (iii)) described herein. In other embodiments, the nozzles (e.g., nozzles 24 and/or 34) and sidewalls 25, 27 may be configured to move (e.g., pivot) collectively within slots in the inner and outer diaphragm segments 20, 22, respectively.

Turning to FIG. 3, a schematic view of a steam turbine system 30 is shown according to embodiments of the invention. As shown, in some embodiments, the steam turbine system 30 may include a control system 32, coupled to the steam turbine 10 (e.g., via wireless, hard-wired and/or electro-mechanical means). In some embodiments, movement of the nozzles (e.g., nozzles 24 and/or 34) is actuated via a control system 32, which may be an electro-mechanical control system configured to provide commands to a mechanical device (e.g., a lever or actuator for each individual nozzle, or for groups of nozzles, a sliding track-based system, a pneumatic system, hydraulic system, electric system, manual system, electro-hydraulic system, etc.). The control system 32 may be part of (or configured to interact with) a conventional steam turbine control system (not shown), and may include associated user interface controls such that the pivoting nozzles 24, 34 (and/or sidewalls 25, 27) may be actuated (e.g., pivoted, rotated, etc.) via a command from a human operator. However, in some embodiments, actuation of the pivoting nozzles (e.g., nozzles 24 and/or 34) may be performed via pre-programmed commands that respond to pre-determined load conditions, e.g., part load (less than 50% rated power), low part load (less than 30% rated power), and/or increased condenser pressure. For example, depending on the optimized conditions and the type of condenser cooling means, this increase in pressure could be an increment as small as 0.5 inches Hg absolute above optimal design conditions for a last stage bucket. In other cases, such as for air cooled condensers, this increase may be greater, e.g., several inches Hg absolute, or the additive combination of both part load and increased condenser pressure above.

FIG. 4 shows a two-dimensional cross-sectional view of a portion of a low pressure steam turbine last stage 400, along with a diffuser 430 according to embodiments of the invention. As shown, the last stage 400 can include a last stage nozzle 410 and a last stage bucket 420, which is configured to rotate along with a rotor in the low pressure steam turbine (both not shown) as is known in the art. The flow from last stage bucket 420 enters the diffuser section 430 of the steam turbine to be routed to the condenser, as indicated by arrows in FIG. 4. As described with reference to the nozzles 24, 34 herein, the last stage nozzle 410 is adjustable to modify a fluid flow within the last stage 400, or within diffuser 430 downstream of the last stage 400, during operation of the steam turbine. In some cases, the last stage nozzle 410 can pivot about one or more axes, e.g., axes i-i, ii-ii, and/or iii-iii. However, the last stage nozzle 410 can be adjusted in any manner described herein, such as those described with reference to nozzles 24, 34.

In some cases, actuation of the pivoting nozzles (e.g., nozzles 24, 34 and/or last stage nozzle 410) may be implemented when the last stage annulus velocity of the steam turbine drops below a mechanical or performance threshold. Turning to FIG. 5, an example graph 500 of an exhaust loss curve 510 is shown for illustrative purposes only. As shown, the graph 500 includes a low pressure steam turbine's exhaust loss plotted against a last stage bucket's annulus velocity, shown as an exhaust loss curve 510. Also illustrated is an operating region 520, which denotes a region of dramatic exhaust losses as the annulus velocity of the last stage bucket decreases. That is, in this operating region 520, a small drop in annulus velocity of the last stage bucket contributes disproportionately to an increase in exhaust losses in the low pressure steam turbine. In order to combat these effects, aspects of the invention allow for adjusting of the last stage nozzle (as described herein) when the last stage bucket's annulus velocity drops below a certain threshold. In some cases, that threshold can be approximately 450 feet/second, or where exhaust losses start exceeding 10 BTUs/lbm. Additionally, adjustment of the last stage nozzles could also be dictated by mechanical considerations (e.g., windage, overheating, bucket flutter/instability), which can be tied to another annulus velocity threshold, e.g., approximately 300 feet/second or less of annulus velocity. Additionally, combinations of higher back-pressures from the condenser could contribute to the concerns noted above, which could modify the annulus velocity thresholds noted.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

1. A low pressure steam turbine section comprising: a rotor having: a rotor body; and a plurality of bucket stages disposed axially along the rotor body, the plurality of bucket stages including a last bucket stage; and a nozzle assembly at least partially surrounding the rotor, the nozzle assembly including a plurality of nozzle stages corresponding to the plurality of bucket stages, wherein a nozzle in a nozzle stage axially upstream of the last bucket stage is adjustable to modify a fluid flow within the last bucket stage during operation of the steam turbine.
 2. The steam turbine of claim 1, wherein the nozzle is one of a plurality of nozzles in the nozzle stage axially upstream of the last bucket stage, and wherein each of the plurality of nozzles is pivotable about a respective axis.
 3. The steam turbine of claim 1, wherein the nozzle is pivotable about an axis substantially perpendicular with a primary axis of the rotor body.
 4. The steam turbine of claim 1, wherein the nozzle is pivotable about an axis substantially parallel with a primary axis of the rotor body.
 5. The steam turbine of claim 1, wherein the nozzle is pivotable at an angle of approximately 30 degrees.
 6. The steam turbine of claim 1, wherein the nozzle includes a blade having: a first segment; and a second segment operably connected to the first segment along the axis, wherein the first segment is configured to pivot relative to the second segment about the axis.
 7. A low pressure steam turbine system comprising: a diffuser section; a turbine coupled to the diffuser section, the turbine including: a rotor having: a rotor body; and a plurality of bucket stages disposed axially along the rotor body, the plurality of bucket stages including a last bucket stage; a nozzle assembly at least partially surrounding the rotor, the nozzle assembly including a plurality of nozzle stages corresponding to the plurality of bucket stages, wherein a nozzle in a nozzle stage axially upstream of the last bucket stage is adjustable to modify a fluid flow within the last bucket stage or the diffuser during operation of the steam turbine; and a control system operably connected to the nozzle stage, the control system configured to actuate pivoting of the nozzle in response to a predetermined load condition.
 8. The steam turbine system of claim 7, wherein the nozzle is one of a plurality of nozzles in the nozzle stage axially upstream of the last bucket stage, and wherein each of the plurality of nozzles is pivotable about a respective axis.
 9. The steam turbine system of claim 7, wherein the nozzle is pivotable about an axis substantially perpendicular with a primary axis of the rotor body.
 10. The steam turbine system of claim 7, wherein the nozzle is pivotable about an axis substantially parallel with a primary axis of the rotor body.
 11. The steam turbine system of claim 7, wherein the nozzle is pivotable at an angle of approximately 30 degrees.
 12. The steam turbine system of claim 7, wherein the nozzle includes a blade having: a first segment; and a second segment operably connected to the first segment along the axis, wherein the first segment is configured to pivot relative to the second segment about the axis.
 13. The steam turbine system of claim 7, wherein the predetermined load condition is a low part load condition or a high condenser pressure condition.
 14. A low pressure steam turbine apparatus comprising: a diffuser section; a turbine coupled to the diffuser section, the turbine including: a rotor having: a rotor body; and a plurality of bucket stages disposed axially along the rotor body, the plurality of bucket stages including a last bucket stage; and a stator substantially surrounding the rotor, the stator including: a nozzle assembly at least partially surrounding the rotor, the nozzle assembly including a plurality of nozzle stages corresponding to the plurality of bucket stages, wherein a nozzle in a nozzle stage axially upstream of the last bucket stage is adjustable to modify a fluid flow within the last bucket stage or the diffuser during operation of the steam turbine.
 15. The low pressure steam turbine apparatus of claim 14, wherein the nozzle is one of a plurality of nozzles in the nozzle stage axially upstream of the last bucket stage, and wherein each of the plurality of nozzles is pivotable about a respective axis.
 16. The low pressure steam turbine apparatus of claim 14, wherein the nozzle is pivotable about an axis substantially perpendicular with a primary axis of the rotor body.
 17. The low pressure steam turbine apparatus of claim 14, wherein the nozzle is pivotable about an axis substantially parallel with a primary axis of the rotor body.
 18. The low pressure steam turbine apparatus of claim 14, wherein the nozzle is pivotable at an angle of approximately 30 degrees.
 19. The low pressure steam turbine apparatus of claim 14, wherein the nozzle includes a blade having: a first segment; and a second segment operably connected to the first segment along the axis, wherein the first segment is configured to pivot relative to the second segment about the axis.
 20. The low pressure steam turbine apparatus of claim 14, wherein the nozzle is adjacent to the last stage bucket. 